Treatment of carbon fibres and composite materials including such fibres



United States Patent US. Cl. 260-37 6 Claims ABSTRACT OF THE DISCLOSUREThe invention comprises a process of treating the surface of carbonfibres by heating the fibre in an oxidising atmosphere at not more than1000 C. for sufiicient time to produce a weight loss of between 0.05 andpreferably not more than 6.0 percent from the fibres, the processenabling a pitted fibre surface to be obtained enabling a good bond tobe obtained between such treated fibres and a supporting matrix in acomposite material.

This invention relates to carbon fibres of the type'suitable for use asa reinforcement in a composite material.

An example of the type of carbon fibre to which the invention relatesare those disclosed in co-pending patent application Ser. No. 449,320,now U.S. Patent Ser. No. 3,412,062, issued Nov. 19, 1968.

As disclosed in this co-pending application it has been proposed to usecarbon fibres disclosed therein as a reinforcement in a compositematerial which comprises a reinforcement of carbon fibres in a matrix ofmaterial such as an epoxy, polyester or Friedel-Crafts type resin. ByFriedel-Crafts type resin is meant a resin formed from an aromaticcompound with an-aromatic linking agent which has two groups, such asmethoxymethyl or chlorornethyl, attached to an aromatic nucleus, bymeans of a polycondensation reaction involving the nuclear hydrogenatoms. The resins are described in more detail in the literature such asthe Transactions and Journal of the Plastics Institute (London) Volume32, N0. 101, pages 298-302 (-1964). Carbon fibers produced according toUS. Patent No. 3,412,062, are of the high strength, high modulus typehaving. an ultimate tensile strength of at least 100x10 pounds persquare inch and a Youngs modulus of at least 16x 10 pounds per squareinch parallel to the fiber axis.'

The strength of such a composite material, and in particular the shearstrength parallel to the fibres in cases where there is selectiveorientation of the fibres, is dependent to some extent on the nature ofthe bond achieved between the reinforcing fibres and the matrix.

It has been suggested that the strength of the bond between the fibresand the matrix may be improved by treatment of the fibres which wouldmodify the surface by a process such as oxidation. However, some surfacetreatments would be likely to reduce the strength of the fibresthemselves to such an extent that there was little or no overall gain instrength of the composite material.

We have, however, discovered a controlled surface treatment of thecarbon fibres which has little, if any, detrimental effect on theirstrength but which nevertheless enables composite material ofconsiderably enhanced shear strength to be produced.

Accordingly the present invention is concerned with the provision of aprocess of treating carbon fibre to obtain fibre surface characteristicswhich enable an improved bond to be obtained between such treated fibresand a supporting matrix in a composite material, and, with compositematerial incorporating such treated fibre as a reinforcement.

A process of treating carbon fibres according to the present inventioncomprises heating carbon fibres in an oxidising atmosphere at. such atemperature and fora limted period of time sufiicient to produce a lossof weight by the fibres of at 1east'0 05 percent and preferably not morethan 6.0 percent from the fibres.

The heat treatment may most conveniently take place in air in which casethe temperature at which heating takes place with not exceed 1000 C. andwill more normally be within the range of from 350-850 C.

The individual fibres must be suificiently separated during the heattreatment process and the temperature, period of heating and gas flowover the fibres must be much that all the fibres are effectivelysubjected to the surface oxidation treatment.

An oxygen rich or pure oxygen atmosphere, or, an atmosphere containingan oxide of nitrogen may be used.

forcement of carbon fibre which has been treated by -a process disclosedabove.

Several examples will now be given:

EXAMPLE 1 Carbon fibres from a batch with a mean strength of 250x10 lb.per square inch of a type produced in accordance with copendingapplication Ser. No. 449,320 and which had been heat treated at from2500-2600 C. were heated for 1 /2 hours at 550 C. and had a weight lossof 0.9 percent. 20 fibres were tested separately for tensile strengthafter this oxidation and had a mean strength of 253 X 10 lb. per squareinch.

EXAMPLE 2 Using carbon fibres of the same type disclosed in and astreated as in Example 1, but using an epoxy resin in place of thepolyester resoin as a matrix, it was found that the shear strength ofthe composite parallel to the fibre axes using the treated carbon fibrereinforcement was 9500-10500 pounds force per square inch as compared to2500-3000 pounds force persquare inch using untreated carbon fibres.

EXAMPLE '4 68.8 g. and 89.75 g. of carbon fibres of the type disclosedin Example 1 and which had been heat treated at 25002600 C. of averagediameter 7.4 microns and of length 14 inches were placed, respectively,into two openended ceramic tubes 14 inches long by 3 inches diameter.

The tubes were placed on refractory bricks in a furnace so that thetubes were in the centre region of the furnace.

The temperature in the furnace was raised to 600 C. and kept at thistemperature for one hour and at the same time a flow of oxygen of 4litres per minute was passed through the furnace.

After cooling down the two bundles of fibres were weighed.

Weight loss of fibres from the first tube was 2.44 percent.

Weight loss of fibres from the second tube was 2.54 percent.

Percent weight Shear strength,

loss of fibres sq. in. X10

EXAMPLE 6 This was in all ways similar to that of Example 5 except thatthe samples were from a different batch of similar fibres. The resultswere as follows:

Percent weight Shear strength, loss of fibres sq. in. X

Time of heating in air at 600 C.:

It is to be noted that at the values of shear strength given thecomposite specimens failed in different manner than interlaminar shearindicating that the ultimate shear strength was greater than the figuresquoted.

EXAMPLE 7 In this case separate 7 inch long samples of carbon fibresproduced in accordance with the disclosure of copending application Ser.No. 449,320 and which had been heat treated for 1 hour at 1500 C. in aninert atmosphere to produce carbon fibres of 8 microns diameter havinghigh strength and high strain were heated in air at the temperaturesshown in the following table and with the resulting percentage weightloss and interlaminar shear strength when incorporated in a matrix:

Sheer Strength, lb./sq. in

Percent Weight loss after oxidation Temperature, C.

Time of heating in air:

It is to be noted with particular reference to Examples 5, 6 and 7 abovethat the effect of the oxidation treatment even for a relatively shortperiod, in one case only 30 minutes, is to almost double theinterlaminar shear strength.

EXAMPLE 8 Carbon fibres of the type disclosed in Example 1 and which hadbeen heat treated at from 2500-2600" C. were heated in air at 500 C. for400 minutes, The treated fibres were found to have a weight loss of 1.22percent and when incorporated in a resin matrix were found to have ashear strength greater than 8.3 x 10 lb. per square inch.

Electron microscopic examination of examples of oxidised carbon fibresshow a pitted surface and it is believed that the higher interlaminarshear strength obtained results from the pitted surface providing a goodkey for the matrix.

The pits range in size from 300 to 400 angstroms at low oxidation weightloss up to nearly 1000 angstroms when the pits coalesce into groovesrunning parallel to the longitudinal fibre axis.

In Examples 2, 3, 5, 6, 7 and 8 above the ratio of carbon fibre tomatrix material in all cases was approximately 62 percent by weight.

We claim:

1. A process of treating carbon fiber having an ultimate tensilestrength of at least 10 pounds per square inch and a Youngs modulusparallel to the fibre axis of at least 16X 10 pounds per square inch toimprove the bonding characteristics of said fibre to a resin matrixcomprising heating the fibre in an oxidizing atmosphere at a temperatureof not more than 1000 C. for a time sufficient to provide a pittedsurface on said fibres and to produce a weight loss of from 0.05 to 6percent by weight based on the weight of the carbon fibre.

2. A process of treating carbon fibres as claimed in claim 1 whereincarbon fibres are heated in air at a temperature of at least 350 C.

3. A process according to claim 1 wherein the carbon fibres are heatedin oxygen for about 1 hour at a temperature within the range of from450600 C.

4. Carbon fibre having improved bonding characteristics to a resinmatrix comprising carbon fibre having an ultimate tensile strength of atleast l00 10 pounds per square inch and a Youngs modulus parallel to thefibre axis of at least 16X 10 pounds per square inch, the surface ofsaid fibre being pitted with a plurality of microscopically visible pitsproduced in said surface representing a weight loss of said fibre offrom about 0.05 to 6 percent by weight based on the weight of the carbonfibre.

5.. A composite material comprising a cured resin matrix containing aplurality of carbon fibres as claimed in claim 4 disposed therein.

6; A composite material as claimed in claim 5 wherein the resin matrixcomprises a resin selected from the group consisting of a polyester,epoxy and Friedel-Crafts type resin.

.References Cited UNITED STATES PATENTS 2,796,331 6/1957 Kauffman at al.2,799,915 7/1957 Barnett et al.

3,053,775 9/1962 Abbott 252-421 OTHER REFERENCES Schmidt et al.,Chemical Engineering Progress, vol. 58, No. 10, October 1962, pp. 4050.

Schmidt et al., Filamentous Carbon and Graphite, Technical ReportAFML-TR-65-l60, August 1965, pp. 11-13.

ALLAN LIEBERMAN, Primary Examiner U.S. Cl. X.R. 23-2091; 106-307; 26040

